UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 7, 1993
NRC INFORMATION NOTICE 93-92: PLANT IMPROVEMENTS TO MITIGATE COMMON
DEPENDENCIES IN COMPONENT COOLING WATER SYSTEMS
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to potential problems resulting from common
dependencies in component cooling water (CCW) systems. Such dependencies on
loss of the CCW system were found to be contributors to the core damage
frequency at several nuclear power plants. It is expected that recipients
will review the information for applicability to their facilities and consider
actions, as appropriate, to avoid similar problems. However, suggestions
contained in this information notice are not NRC requirements; therefore, no
specific action or written response is required.
Background
On November 23, 1988, the NRC issued Generic Letter 88-20, "Individual Plant
Examination for Severe Accident Vulnerabilities," requesting that each nuclear
power plant perform a systematic examination to identify any plant-specific
vulnerabilities to severe accidents and report the results to the Commission.
As stated in the letter, licensees are expected to expeditiously correct any
identified vulnerabilities. This notice provides insights gained from the
individual plant examination (IPE) process regarding improvements in plant
design or operational practices and procedures to mitigate potential problems
resulting from common dependencies in CCW systems.
Description of Circumstances
CCW systems provide cooling for reactor coolant pump (RCP) seals, and in many
cases also supply cooling water to emergency core cooling system (ECCS)
components. Consequently, failure of the CCW system could cause a loss of RCP
seal cooling leading to an RCP seal loss of coolant accident (LOCA) and could
also disable the necessary accident mitigation systems. Several licensees
have identified these common dependencies as contributors to the total plant
core damage frequency in their IPEs. The following cases illustrate the
measures taken by several utilities to mitigate the consequences of such
accidents.
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Turkey Point Units 3 and 4
The Turkey Point IPE documented a potential accident sequence caused by an
unisolable rupture of the CCW system that could occur while the B charging
pump is out of service. In this scenario, cooling to the RCP seals would be
lost, as would cooling to the A and C charging pumps. If the charging pumps
are run without CCW, they are predicted to fail. This scenario would,
according to the IPE, cause an RCP seal LOCA because all means of providing
seal cooling would be lost from either the normal CCW system supply or the
alternate seal injection supply from the charging pumps. Additionally,
because the CCW system supplies cooling to the low and high head safety
injection pumps, these pumps also would fail. Thus, an unmitigated RCP seal
LOCA would occur.
This finding led the licensee to modify the A and C charging pumps in 1991 so
that the service water system can be aligned as an alternate cooling water
supply to any of the charging pumps. (The B charging pump had been similarly
modified in 1976.) Thus, a loss of the CCW system alone would not disable all
of the ECCS components required for LOCA mitigation. This modification
reduced the contribution of this sequence to the total core damage frequency.
However, during an NRC Operational Readiness Assessment Team inspection at the
facility, procedural deficiencies were identified that could have prevented
this modification from performing its intended function. The licensee
corrected the deficiencies.
H.B. Robinson Steam Electric Plant, Unit 2
During development of the Robinson IPE, the licensee identified an accident
sequence that could be initiated by a total loss of the CCW system. The
configuration at Robinson is similar to that at
Turkey Point in that the CCW system supplies cooling to both the RCP seals and
the charging pumps. Thus, loss of the CCW system could lead to an RCP seal
LOCA coincident with the inability to provide high pressure makeup.
This discovery led the licensee to implement procedural revisions that allow
the alignment of an alternate cooling supply to the charging pumps by
connecting the firewater system to existing fittings on the charging pump
couplings. This procedural revision reduced the estimated core damage
frequency contribution for the sequence.
Watts Bar Nuclear Plant, Unit 1
Before performing the IPE, the licensee modified a procedure to align a backup
cooling water supply to the 1A-A centrifugal charging pump (CCP) via the
emergency raw cooling water (ERCW) system. The CCPs depend on the CCW system
for normal cooling to the pump lube oil and gear oil coolers. The CCW system
also supplies cooling to the residual heat removal pumps, safety injection
pumps, containment spray pumps, and the
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RCP thermal barrier coolers. In the IPE, the licensee credited operator
actions associated with aligning the ERCW backup during a total loss of CCW
system scenario. The IPE indicated that alignment of the ERCW system to the
CCP reduced the incremental core damage frequency.
A recent NRC Emergency Operationg Procedure team inspection at the facility
identified procedural deficiencies that could have prevented this alignment
from performing its intended function. The licensee is developing measures to
correct these deficiencies.
Diablo Canyon Nuclear Power Plant, Units 1 and 2
At Diablo Canyon Units 1 and 2, the CCW system provides cooling to the
charging pumps, which supply cooling water to the RCP seals. While developing
the probabilistic risk assessment, the licensee discovered that loss of the
CCW system could result in loss of the charging pumps and the RCP seals and
could lead to an RCP seal LOCA. To mitigate this consequence, the licensee
provided an alternate means of cooling the charging pumps from the fire water
system on loss of the CCW system. Hose connections and dedicated hoses were
provided and operational procedures were changed to facilitate this alternate
means of cooling. As a result, the licensee indicated that the core damage
frequency associated with loss of the CCW system was reduced.
Donald C. Cook Nuclear Plant, Units 1 and 2
The CCW system for the Cook Nuclear Plant provides cooling to the RCP seals
and various primary plant components. Loss of the CCW system leading to an
RCP seal LOCA was identified as a dominant contributor to the core damage
frequency. The licensee is currently investigating changes to operational
procedures to instruct the operator to open the cross-tie valve of the
chemical and volume control system of one unit to the opposite unit early in
the accident response to provide RCP seal cooling and to prevent seal damage.
Discussion
As indicated above, several licensees have identified, through the IPE
process, that a common dependency in the CCW system to provide cooling for RCP
seals and ECCS components could result in a significant contribution to the
core damage frequency. Licensees found that changes in plant operational
practices and procedures or cost-effective improvements in design could reduce
the calculated core damage frequency.
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This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
the technical contacts listed below or the appropriate Office of Nuclear
Reactor Regulation project manager.
/s/'d by BKGrimes
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: J. L. Shackelford, RII
(404) 331-0338
M. T. Drouin, RES
(301) 492-3917
Attachment:
List of Recently Issued NRC Information Notices
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